I made a cake/cupcakes for a large party last weekend that had to be cancelled due to rain.  So - they ate the cupcakes, stuck the cake in the fridge & rescheduled the party for a week away.  

She has now ordered another 2 dozen cupcakes for the new party date and ordered a coordinating, smaller cake to go with the still pretty, but no longer fresh, cake she has sitting in her fridge.  The new cake will be a ruffle, buttercream cake with a gumpaste flower attached to the side/top.  I have flowers left over from the last cake and would love to use them for the new cake but I did not wire them.  

Will a single flower stick to the side of a ruffle buttercream cake?

You could experiment on the side of a cake pan.  Ruffle some buttercream up the side and put a flower on and see how well it holds...

Probably not, unless it has some kind of support sticking into the cake. What kind of flower is it? You could take two coffee stirs and insert them into the side of the cake to make a little "rack" to rest the flower on to make sure it doesn't fall off.

Hmmmm..... Anyway to attach a wire to an already made flower?

AIf your flowers have a central ball/bud in, then one thing you can try is to heat your flower wires up (strip off paper if covered wires) over a lighter flame and stick them into the ball? It's a bit tricky - the wire will get very hot so if you do try it, so please be very careful! I've seen this work when a flower has fallen off a wire while being made, although not on a fully dried flower so not entirely sure if this will still work?

Otherwise I suspect you might have to make from scratch - I don't think unwired flowers will stick to buttercream :/

Good luck!

To insert into a cake I wouldn't do wires...Not food safe. What kind of flower is it? How is it built? if we know that it would help.

AWires should be fine, as long as you use a flower pick first (standard practise in the uk). The pick goes into the cake, then you insert the wires into the pick, which is normally filled with flower paste or RI to hold the wires in place - no nasty wires going directly into the cake this way :)